In a high-pressure mercury lamp, mercury that is a light-emitting material is enclosed in a discharge space in a discharge vessel, and a pair of electrodes is arranged so that tips of the electrodes are in opposition to each other in the discharge space. Some of such high-pressure mercury lamps have a discharge space in which halogen in addition to mercury is enclosed, in order to reduce electrode consumption using a function of a halogen cycle that occurs while a discharge of the high-pressure mercury lamp is maintained.
In such a high-pressure mercury lamp using the function of the halogen cycle, an applied voltage to the high-pressure mercury lamp, an enclosed amount of mercury or halogen, a distance between electrodes, and the like are determined so that the high-pressure mercury lamp is in a proper state in which the halogen cycle effectively functions while a discharge of the high-pressure mercury lamp is maintained. Note that as a material of an electrode, tungsten is commonly used.
The above high-pressure mercury lamp (hereinafter, referred to as “lamp”) is operated (including a case in which a discharge of the high-pressure mercury lamp is maintained) by a constant current control until a lamp voltage reaches a predetermined voltage. At an instant when the lamp voltage reaches the predetermined voltage, the constant current control is switched to a constant power control to maintain a power supplied to the lamp at that instant as a target power value.
When an accumulated time during which a discharge of a lamp is maintained (hereinafter, referred to as “accumulated operating time”) becomes longer, a state of the halogen cycle deviates from the proper state. Due to this deviation, the halogen cycle does not effectively function, and tungsten, which is to be evaporated from an electrode and accumulated on the electrode, is adhered to an inner surface of a discharge vessel that forms a discharge space (blackening phenomenon).
If the discharge is further maintained, heat is accumulated in tungsten which is adhered to the discharge vessel, and a glass in a part of the discharge vessel, to which tungsten is adhered, is recrystallized and devitrified. Because of the devitrification phenomenon and the blackening phenomenon, emission of light from the discharge space to outside is shielded, and an amount of the emission of light from the lamp is reduced, which leads to shortening of a life of the lamp (it is regarded as the end of a life of the lamp when luminance becomes about 50% of initial luminance).
As a technique of extending a life of a lamp, the following technique (patent document 1) is disclosed. A point at which deterioration of the lamp starts has been obtained by an experiment in advance. When an accumulated operating time reaches the point (predetermined time), a target power value for the constant power control is reduced.
Patent Document 1: Japanese Published Patent Application No. 2003-131324